Network Working Group M. Boutier
Internet-Draft J. Chroboczek
Updates: 6126bis IRIF, University of Paris-Diderot
(if approved) June 15, 2017
Intended status: Standards Track
Expires: December 17, 2017
Source-Specific Routing in Babel
draft-boutier-babel-source-specific-02
Abstract
This document describes an extension to the Babel routing protocol to
support source-specific routing.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 17, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Boutier & Chroboczek Expires December 17, 2017 [Page 1]
Internet-Draft Source-Specific Routing in Babel June 2017
Table of Contents
1. TODOs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction and background . . . . . . . . . . . . . . . . . 3
3. Data Structures . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. The Source Table . . . . . . . . . . . . . . . . . . . . . 3
3.2. The Route Table . . . . . . . . . . . . . . . . . . . . . 3
3.3. The Table of Pending Requests . . . . . . . . . . . . . . 4
4. Data Forwarding . . . . . . . . . . . . . . . . . . . . . . . 4
5. Protocol Operation . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Source-specific messages . . . . . . . . . . . . . . . . . 5
5.2. Route Acquisition . . . . . . . . . . . . . . . . . . . . 5
5.3. Wildcard requests . . . . . . . . . . . . . . . . . . . . 6
6. Backwards compatibility . . . . . . . . . . . . . . . . . . . 7
6.1. Loop-avoidance . . . . . . . . . . . . . . . . . . . . . . 8
6.2. Starvation and Blackholes . . . . . . . . . . . . . . . . 8
7. Protocol Encoding . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Source Prefix sub-TLV . . . . . . . . . . . . . . . . . . 9
7.2. Source-specific Update . . . . . . . . . . . . . . . . . . 9
7.3. Source-specific (Route) Request . . . . . . . . . . . . . 10
7.4. Source-Specific Seqno Request . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security considerations . . . . . . . . . . . . . . . . . . . 10
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
10.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Boutier & Chroboczek Expires December 17, 2017 [Page 2]
Internet-Draft Source-Specific Routing in Babel June 2017
1. TODOs
o Source Prefix sub-TLV type: TBD
o check references (Section) for BABEL in 6126bis
2. Introduction and background
Source-specific routing (other known as Source Address Dependant
Routing, SAD Routing or SADR) is an extension to traditional next-hop
routing where packets are routed according to both their destination
and their source address. This document describes the source-
specific routing extension to the Babel routing protocol as defined
in 6126bis [BABEL]. It notably requires the sub-TLV mandatory bit.
Background information about source-specific routing is provided in
[SS-ROUTING].
3. Data Structures
This extension adds some data to the data structures maintained by a
Babel node.
3.1. The Source Table
Every Babel node maintains a source table, as described in [BABEL],
Section 3.2.5. A source-specific Babel node extends this table with
the following field:
o the source prefix (sprefix, splen) specifying the source address
of packets to which this entry applies.
If a source table entry has a zero length source prefix (splen equals
to 0), then the entry is a non-source-specific entry, and is treated
just like a source table entry defined by the original Babel
protocol.
With this extension the route entry contains a source which itself
contains a source prefix. Notwithstanding the accidental similarity
in their names, these are two very different concepts, and should not
be confused.
3.2. The Route Table
Every Babel node maintains a route table, as described in [BABEL],
Section 3.2.6. With this extension, this table is indexed by the
5-tuple (prefix, plen, source prefix, source plen, router-id)
Boutier & Chroboczek Expires December 17, 2017 [Page 3]
Internet-Draft Source-Specific Routing in Babel June 2017
obtained from the associated source table entry.
If a route table entry has a zero length source prefix, then the
entry is a non-source-specific entry, and is treated just like a
route table entry defined by the original Babel protocol.
3.3. The Table of Pending Requests
Every Babel node maintains a table of pending requests, as described
in [BABEL], Section 3.2.7. A source-specific Babel node extends this
table with the following entry:
o the source prefix being requested.
4. Data Forwarding
In next-hop routing, if two routing table entries overlap, then one
is necessarily more specific than the other; the "longest prefix
rule" specifies that the most specific applicable routing table entry
is chosen.
With source-specific routing, there might no longer be a most
specific applicable prefix: two routing table entries might match a
given packet without one necessarily being more specific than the
other. Consider for example the following fragment of a routing
table:
(2001:DB8:0:1::/64, ::/0, A)
(::/0, 2001:DB8:0:2::/64, B)
This specifies that all packets with destination in 2001:DB8:0:1::/64
are to be routed through A, while packets with a source in 2001:DB8:
0:2::/64 are to be routed through B. A packet with source 2001:DB8:0:
2::42 and destination 2001:DB8:0:1::57 matches both rules, although
neither is more specific than the other. A choice is necessary, and
unless the choice being made is the same on all routers in a routing
domain, persistent routing loops may occur.
A Babel implementation MUST choose routing table entries by using the
so-called destination-first ordering, where a routing table entry R1
is preferred to a routing table entry R2 when either R1's destination
prefix is more specific than R2's, or the destination prefixes are
equal and R1's source prefix is more specific than R2's. (In more
formal terms, routing table entries are compared using the
lexicographic product of the destination prefix ordering by the
source prefix ordering.)
Boutier & Chroboczek Expires December 17, 2017 [Page 4]
Internet-Draft Source-Specific Routing in Babel June 2017
In practice, this means that a source-specific Babel implementation
must take care that any lower layer that performs packet forwarding
obey this semantics. In particular:
o If the lower layers implement the destination-first ordering, then
the Babel implementation MAY use them directly;
o If the lower layers can hold source-specific routes, but not with
the right semantics, then the Babel implementation MUST
disambiguate the routing table by using a suitable disambiguation
algorithm (see [SS-ROUTING] for such an algorithm);
o If the lower layers cannot hold source-specific routes, then a
Babel implementation MUST silently ignore any source-specific
routes.
5. Protocol Operation
This extension does not fundamentally change the operation of the
Babel protocol. We only described the fundamental differences
between the original protocol and the extension in this section. The
other mechanisms described in [BABEL] (Section 3) may be infered by
using pairs of (destination, source) prefixes instead of just
(destination) prefixes.
5.1. Source-specific messages
A route of this extension with a zero-length source prefix is the
same than a route without source prefix (a route of the classical
Babel). In both of the cases, packets are accepted independantly of
their source address. Thus, a route is said source-specific only if
its source prefix has a non-zero length.
Three messages are used to communicate informations on routes:
Updates, Route Requests and Seqno Requests. With this extension,
these messages carry an additionnal source prefix if (and only if)
the corresponding route is source-specific. More formally, an
Update, a Route Request and a Seqno Request MUST carry a source
prefix if they concern a source-specific route (non-zero length
source prefix) and MUST NOT carry a source prefix otherwise (zero
length source prefix). A message which carry a source prefix is said
source-specific.
5.2. Route Acquisition
When a non-source-specific Babel node receives a source-specific
update, it just ignores it.
On receipt of a source-specific update (id, prefix, source prefix,
Boutier & Chroboczek Expires December 17, 2017 [Page 5]
Internet-Draft Source-Specific Routing in Babel June 2017
seqno, metric), a source-specific Babel node behaves as described in
[BABEL] Section 3.5.4 though indexing entries by (neigh, id, prefix,
source prefix). When a source-specific Babel node receives a non-
source-specific update, it MUST consider this update as carrying a
zero length source prefix.
5.3. Wildcard requests
TODO: behaviour to be defined.
5.3.1. Proposal 1
The original Babel protocol states that when a node receives a
wildcard route request, it SHOULD send a full routing table dump.
This extension does not change this statement: a source-specific node
SHOULD send a full routing table dump when receiving a wildcard
request.
Source-specific wildcard requests does not exist: a wildcard request
SHOULD NOT carry a source prefix.
5.3.2. Proposal 2
We assume that a mandatory sub-TLV has a corresponding non-mandatory
sub-TLV. This proposal is like Proposal 3 but instead of having
multiple wildcard request TLVs, one for each kind of routes
understood, we use one wildcard request with sub-TLVs corresponding
to the extension. To have a full routing table dump, a node sends a
wildcard requests with a non-mandatory Source sub-TLV.
A source-specific node SHOULD always attach a non-mandatory Source
sub-TLV to its wildcard requests.
This proposal has been rejected because it implies to share the space
of non-mandatory and mandatory sub-TLVs.
5.3.3. Proposal 3
The Babel protocol provides the ability to request a full routing
table dump by sending a "wildcard request", a route request with the
AE field set to 0. As the original protocol has no source-specific
routes, such a request may only concern non-source-specific routes.
This extension does not modify the semantics of wildcard requests in
that sense: a wildcard request prompts the receiver to send its non-
source-specific routes only, and a Babel node SHOULD NOT send any
source-specific updates in reply to a wildcard request.
To obtain a dump of the source-specific routes, a source-specific
Boutier & Chroboczek Expires December 17, 2017 [Page 6]
Internet-Draft Source-Specific Routing in Babel June 2017
wildcard request MUST be used. A source-specific wildcard request is
a wildcard request carrying a zero length source prefix.
When a node receives a source-specific wildcard request, it SHOULD
send a dump of its routes which are source-specific "only". It
SHOULD NOT send any non-source-specific routes in reply to a source-
specific wildcard request. It SHOULD NOT send any source-specific
routes which are under the effect of a future extension. Such
extension should detail how to handle the possible combinations.
In consequence, a node requiring a full routing table dump must send
both a non-source-specific wildcard request and a source-specific
wildcard request.
5.3.4. Proposal 4
Wildcard requests are deprecated. Either deprecate it in 6126bis, or
say the following.
A node receiving a wildcard request SHOULD ignore it.
This proposal has been rejected because wildcard requests speeds up
the convergence of the network on boot. This is considered
important.
5.3.5. Note on Overhead between (1) and (3)
Sending one wildcard request (1) instead of a few something-specific
wildcard requests (3) in a negligible gain.
Non-source-specific nodes sending requests to source-specific nodes
may reduce the global overhead with (3). But, if the network has no
source-specific route, there is no overhead to reduce; if there is
only a few source-specific routes (like in a home network), the
overhead would be negligible. Thus, the interesting case is when
there is a lot of source-specific routes.
We can imagine a network with a source-specific backbone announcing a
default route and catching all trafic. Good old routers not
supporting this extensions would be put at some backbone leafs. Is
sbabeld part of that use case ?
Couldn't we just send a Route Request for *default* ?
6. Backwards compatibility
The protocol extension defined in this document is, to a great
Boutier & Chroboczek Expires December 17, 2017 [Page 7]
Internet-Draft Source-Specific Routing in Babel June 2017
extent, interoperable with the base protocol defined in [BABEL] (and
all its known extensions). More precisely, if non-source-specific
routers and source-specific routers are mixed in a single routing
domain, Babel's loop-avoidance properties are preserved, and, in
particular, no persistent routing loops will occur.
TODO: Should we put a warning to say it's not the case with the
Experimental Track Babel ?
6.1. Loop-avoidance
The extension defined in this protocol uses a new Mandatory sub-TLV
to carry the source prefix information. As discussed in Section 4.4
of [BABEL], this encoding ensures that non-source-specific routers
will silently ignore the whole TLV, which is necessary to avoid
persistent routing loops in hybrid networks.
Consider two nodes A and B, with A source-specific announcing a route
to (D, S). Suppose that B ignores the source prefix information when
it receives the update, and reannounces it as D. This is reannounced
to A, which treats it as (D, ::/0). Packets destined to D but not
sourced in S will be forwarded by A to B, and by B to A, causing a
persistent routing loop:
(D,S) (D)
(D,::/0)
6.2. Starvation and Blackholes
In general, discarding of source-specific routes by non-source-
specific routers will cause routing starvation. Intuitively, unless
there are enough non-source-specific routes in the network, non-
source-specific routers will suffer starvation, and discard packets
for destinations that are only announced by source-specific routers.
A simple yet sufficient condition for avoiding starvation is to build
a connected source-specific backbone that includes all of the edge
routers, and announce a (non-source-specific) default route towards
the backbone. However, introducing such a default route in the
network may in the same time introduce a blackhole. This tradeoff is
let to the administrator.
Boutier & Chroboczek Expires December 17, 2017 [Page 8]
Internet-Draft Source-Specific Routing in Babel June 2017
7. Protocol Encoding
This extension defines a new sub-TLV used to carry a source prefix by
the three following existing messages: Update, Route Request and
Seqno Request.
7.1. Source Prefix sub-TLV
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBD | Length | Source Plen | Source Prefix...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Fields:
Type Set to TBD to indicate a Source Prefix sub-TLV.
Length The length of the body, exclusive of the Type and Length
fields.
Source Plen The length of the advertised source prefix. This MUST
NOT be 0.
Source Prefix The source prefix being advertised. This field's size
is (Source Plen)/8 rounded upwards.
The Source Prefix field's encoding is the same than the Prefix's. It
is defined by the AE field of the corresponding TLV.
Remark that this sub-TLV is a Mandatory sub-TLV. The whole TLV MUST
be ignored if that TLV is not recognized. Otherwise, routing loops
may occur.
7.2. Source-specific Update
The source-specific Update is an Update TLV with a Source Prefix sub-
TLV. It advertises or retracts source-specific routes in the same
manner than routes with non-source-specific Updates (see [BABEL])
except for wildcard updates.
Wildcard updates MUST NOT carry any source prefix. Wildcard updates
(in fact, wildcard retraction) are used when a Babel node stops: a
receiver retracts all routes announced by the announcing node. There
is no use case for source-specific wildcard updates. A source-
specific Babel node receiving a (legacy) wildcard update MUST
retracts all routes it learns from this node (including source-
specific ones).
Contrary to the destination prefix, this extension does not compress
the source prefix attached to Updates. The destination prefix uses
compression as defined in [BABEL] for Updates with Mandatory
Boutier & Chroboczek Expires December 17, 2017 [Page 9]
Internet-Draft Source-Specific Routing in Babel June 2017
extensions.
However, as defined in [BABEL] (Section 4.5), the compression is
allowed for the destination prefix of source-specific routes. Legacy
implementation will correctly update their parser state, while
ignoring the whole TLV afterwards.
7.3. Source-specific (Route) Request
TODO: A source-specific Route Request prompts the receiver to send an
update for a given pair of destination and source prefixes. It MUST
NOT be used to request a full routing table dump. The Source Prefix
sub-TLV of a wildcard source-specific Route Request (Request with AE
equals to 0 and a Source Prefix sub-TLV) MIGHT be ignored: a receiver
MIGHT reply by a full routing table dump.
7.4. Source-Specific Seqno Request
A source-specific Seqno Request is just like a Seqno Request for a
source-specific route. It uses the same mechanisms described in
[BABEL].
8. IANA Considerations
IANA is instructed to add the following entry to the "Babel sub-TLV
Types" registry:
+------+---------------+-----------------+
| Type | Name | Reference |
+------+---------------+-----------------+
| TBD | Source Prefix | (this document) |
+------+---------------+-----------------+
9. Security considerations
The extension defined in this document adds a new sub-TLV to three
TLVs already present in the original Babel protocol. It does not by
itself change the security properties of the protocol.
10. References
10.1. Normative References
[BABEL] Chroboczek, J., "The Babel Routing Protocol", Internet
Draft draft-ietf-babel-rfc6126bis-02, May 2017.
Boutier & Chroboczek Expires December 17, 2017 [Page 10]
Internet-Draft Source-Specific Routing in Babel June 2017
10.2. Informative References
[SS-ROUTING]
Boutier, M. and J. Chroboczek, "Source-Specific Routing",
August 2014.
In Proc. IFIP Networking 2015. A slightly earlier
version is available online from
http://arxiv.org/pdf/1403.0445.
Authors' Addresses
Matthieu Boutier
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13,
France
Email: boutier@irif.fr
Juliusz Chroboczek
IRIF, University of Paris-Diderot
Case 7014
75205 Paris Cedex 13,
France
Email: jch@irif.fr
Boutier & Chroboczek Expires December 17, 2017 [Page 11]